Just figured this out now, but the arial default seems to be because "surprised pikachu" is the first meme listed in popular when you go to make a meme, and somehow the default font is saved as arial on that meme.

There are several other memes that have different font setting, like "two buttons", "mocking spongebob", and "Y U NO" of course (and "roll safe" seems to have NO default, so it mimics the settings of the last meme you were on), but if you type a little bit on a line and then click through the memes, especially before you upload your own image to meme, you can find one that puts the font settings back to the usual allcaps impact.

This won't help on the memes that have arial font set, but it's at least a bit of a workaround.

I mean, my explanation may have been a bit wordy, but it's really simple. If I want to use my own image, I just click on a popular meme with the right font settings and then I'm good to go, I can upload my image.

Font settings have always been saved when switching memes, but now that we added custom default font settings, this had the unintended side effect of not switching back to white/black/impact when selecting a non-customized meme. This part should be fixed now.

Default fonts have been a missing feature for a long time, and sadly/hilariously I think Imgflip is somewhat responsible worldwide for killing some of the original nature of some memes because we lacked some customization ability (and Imgflip might be worldwide #1 meme generator at this point..). For example, Doge originally uses comic sans and multiple colors (imgflip.com/memegenerator/Doge). Comics do better with Arial and no text shadow, because it is MUCH more readable when the font gets small in those tiny chat bubbles. The reason all-caps/impact/black/white is the default is due to the original Advice Animal style which only uses a top and bottom caption on a simple character, and using extreme contrast (white on black) ensures the font is readable on any possible image background. Many memes have evolved far beyond that style now and so people are starting to use a wider variety of font settings.